A variety of locking threaded fasteners or fastening assemblies are known in the art. The goal of such systems is to prevent the male and female components of the fastening assemblies from loosening and coming apart when subjected to vibration, expansion, contraction and other possible movement. Most such systems rely on provision of increased friction to provide improved thread locking. Such systems include thread coatings such as LOCTITE.RTM. thread coatings, and/or other products to create a locking friction between the male and female threads, and/or an increased locking friction between the head of a fastener and the surface of a workpiece to which it is fastened. Some systems employ a mechanical locking system to maintain the respective positions of the male and female fastener components. One example of a known mechanical system is a locking nut, in which is positioned a collar with a plurality of flexible arms extending inwardly; these arms seat in longitudinal channels formed on the outer periphery of the threaded male fastener. Upon engagement of the arms with respective channels, loosening is prevented by preventing relative rotation of the components in a loosening direction. The above described structure is illustrated in U.S. Pat. No. 5,238,342 issued to Stencel.
A typical fastener application in automotive applications using plastic materials involves a metal or plastic screw that is used to secure a component to a plastic workpiece. A problem in such applications is that axial loads resulting from torque are concentrated on a relatively narrow region of the boss surrounding the male screw fastener, causing potential for damage by cracking the workpiece. In many such applications, a metal sleeve or insert is provided in the plastic workpiece to provide for distribution of the force applied in fastening, and to prevent damage by the male fastener to the workpiece. There are significant drawbacks to the use of such inserts. The use of such inserts is expensive, requiring purchase or fabrication of an additional separate component, and the installation thereof in the boss before the screw fastener can be used. This slows the manufacturing speed by requiring the additional manufacturing steps of installing numerous inserts in the plastic workpiece where the fastening will occur. In addition to being expensive, such inserts are prone to pullout from the plastic workpiece at relatively low levels of force. Thus, a component fastened to a plastic workpiece where an insert is used may be less securely installed than would be optimal. For example, in a thermoset plastic, the pullout forces required to remove an insert may actually be less than that needed to remove a screw properly screwed into the workpiece. Finally, the use of inserts impedes recycling of the plastic workpiece in the event the workpiece is disassembled, because the metal sleeve or insert must be drilled out of or punched from the plastic workpiece before the plastic can be recycled.
A common problem encountered in fastener applications used in plastic workpieces arises when the fastener is to operate as a stud onto which another component is to be mounted, and is secured with a nut. One risk is the cross-threading of the nut on the stud, which can result in the nut locking up or seizing on the stud. Continued tightening of a cross-threaded nut can cause turning of the self-tapping threads of the screw, thereby potentially damaging the boss. In the same way, if a nut bottoms out on the stud mounted onto the plastic workpiece, there is the distinct possibility that the nut will strip or otherwise damage the boss.
Therefore, there is a need for a locking screw fastener adapted for use in mounting particularly (though not exclusively) to plastic workpieces; such locking screw fasteners should not require metal inserts or sleeves to be inserted in the workpiece; and they should be adapted to prevent damage to the workpiece in the event of cross-threading or overtorquing of a nut onto a stud portion of the fastener.